1. Field of the Disclosure
The present disclosure is directed towards the field of monitoring of subterranean formations. Specifically, the disclosure is a method and apparatus that uses seismic detectors to identify the location of microseismic events that occur in subsurface formations due to minerals mining, subsurface engineering or hydrocarbon-related production activities, including waste disposal operations.
2. Description of the Related Art
One aspect of hydrocarbon-related reservoir development includes fracturing of the reservoir. Such fracturing operations result in so-called “microseisms” which can be detected by suitable acoustic sensors in the subsurface or at the surface of the earth. This is discussed, for example, in U.S. Pat. No. 6,253,848 to Reimers et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference. Microseismic activity may also occur in secondary recovery operations with CO2 injection or steam injection at the interface between the injected fluid and original fluid in the earth formation. In such a situation, it is possible that the microseismic activity is more likely along pre-existing fractures in the earth formation (where the injected fluid has an easier flow path). Microseismic activity may also occur in waste-disposal operations where wastes are pumped into a formation below an impermeable layer. Microseismic activity may also occur in storage of gas in underground reservoirs and in disposal of CO2 that may be recovered in oil and gas operations. These are examples of microseismic activity resulting from stress changes in the subsurface associated with flow of a fluid. Microseismic activity may also result from stress changes that may not be associated with a fluid flow: these commonly occur in mining operations wherein drilling into a mineral formation produces a change in the stress distribution in the formation.
In prior art methods, the monitoring and hypocenter location of passive microseisms in oil reservoirs is often accomplished by using a single Vertical Seismic Profile (VSP) downhole geophone array to derive information of azimuth, angle of inclination and P, S-phase first arrival times of the events recorded by the 3-C geophones (Oye and Roth, 2003). The term “hypocenter” refers to the point at which the first motion in an earthquake originates. In the context of the present disclosure, the term “hypocenter” refers to a location at which ground motion originates in operations such as a fracturing operation. The uncertainty of the hypocenter location using this kind of single VSP array is generally of the order of tens of meters and often needs many additional conditions to be met (Fabriol, 2001; Phillips et al., 1998; Rutledge et al, 1998). Using this method, the measurement of azimuth and angle of inclination of an event from a hodogram is often severely affected by the quality of P-wave arrivals such as S/N ratio, impulsivity, and the assumed velocity model (Vadale, 1986). Another weakness of the prior art methods is that the amplitudes of P-wave first arrival may be small and hard to detect, making it difficult to locate the origin of the microseismic event. The waste of numerous precious events recorded during a short-term temporary monitoring period of microseismicity could lead to the failure of a whole project. In addition, the measurement and evaluation of azimuth and angle of inclination of signals are often time-consuming and may be inaccurate. These disadvantages will greatly affect the accuracy and efficiency of the data processing and the estimated hypocenter location of microseismic monitoring. 3-C measurements made in a plurality of vertical arrays provide improved results. See U.S. Pat. No. 6,023,657 to Kerekes.
Hypocenter location using P and/or S-wave first arrival times have long been used and proved to be much more accurate than methods using a combination of azimuth, angle of inclination, and time difference between P, and S phases. Obviously, the straight line geometry of a single VSP is not suitable for the application of the arrival time method. An application of such a monitoring location method has been proposed by using multiple VSP arrays (Chen, 2006). In recent years, the emergence of the techniques of deployment of subsurface geophone arrays, has greatly improved the S/N ratio of events. The combination of a VSP and a subsurface array forms a network with geometry suitable for the application of such a hypocenter location method.
There is a need for an accurate and easy to deploy method for identifying the origins of microseismic events in the subsurface. The present disclosure addresses this need.